The invention concerns a method for the control of the properties of the oil layer for oil containing reactive metallic materials, particularly for magnesium or magnesium alloy particles such as granules, chips and turnings.
The normal oil cleaning/removal processes available on the market are based on chemical/mechanical cleaning principles involving use of some kind of reagent. There are two reagents commonly used for the said purposes; one reagent is usually from the tensides group, in particular ethene oxide group-type such as fatty alcohols or ethoxylate. The said reagents reduce surface tension of the oil with the result that the oil leaves the material and forms an emulsion in water. The problem in such a method lies in separation of oil from the oil and water mixture which is necessary before the washing filtrate is allowed to be drained/discarded in nature. Public health authorities demand such filtrate to contain less than 100 ppm of oil before it is to be discarded in drainage. By use of some special reagents and powerful agitation, it is now possible to separate out oil from the said materials as well as from the filtrate down below the limits, but it makes the oil removal process more complex and considerably costly. In addition, the process is not suitable for materials that can not sustain water. Unfortunately, magnesium is such a material and magnesium granules therefore in practice are defined as "danger when wet with water".
The other reagent is of solvent type which dissolves oils/mineral oils from the surface of the materials. In practice, the oil containing materials are treated/sprayed with an excess of solvent at various stages. For making the material practically free of the oil, use of reagents based on light dearomatized hydrocarbon such as heptane and/or methane are preferred. The reagents are then removed from the filtrate in a separate distillation process. However, use of such reagents is extremely dangerous with regard to fire as their flash points are rather low. Besides, these reagents have relatively high vapour pressure at room temperature which creates risk of environmental pollution inside the deoiling plant. In a few deoiling plants one has tried to use chlorinated hydrocarbon reagent; methyl chloride for avoiding danger of fire. Use of such reagent, however, involves the danger of serious pollution problems and is therefore not easily allowed by the local authorities.
In Norwegian patent No. 172 838 a new, effective and simple deoiling process particularly for the removal of oil from oil containing magnesium chips, turnings/magnesium fine materials has been described. The said process is based on removing the oil by evaporation during heating of the material to appropriate temperatures. The rich oil vapours arising in the process are condensed to recover the oil.
Another Norwegian patent application No. 931784 describes a process for the production of pure magnesium or magnesium alloy granules directly from molten metal. The said process produces oil containing metal granules from which oil has to be removed to extremely low level before the product can be sold on the market. For oil removal, one can of course use the above mentioned process described in Norwegian patent No.172838 where the material is first subjected to a centrifuge treatment to remove the excess oil and thereafter it is heated to reduce the oil concentration to very low levels, below 0.1%. However, as far as deoiling of the granules produced directly from liquid metal through the said process is concerned, the deoiling process described in the said patent may not be very effective. The reasons are:
The said granulation process of liquid magnesium is based on using a suitable oil bath as a cooling medium for liquid magnesium droplets. From the point of view of the metal granulation process, it is very important that the cooling oil has certain specific properties. The oil must be of non-polar type having very low concentration of aromatics both ordinary aromatics as well as polycrystalline aromatics. It must also have a relatively high flash point, preferably higher than 180.degree. C., so as to avoid generation of flash from the oil surface, and it should have very low vapour pressure in the temperature range below 200.degree. C. This is important for maintaining atmosphere above the oil bath practically free of oil vapours. The oil must also have reasonably good resistance to oxidation and decomposition so it can be used continuously in the process for a very long period. The oil should also have low viscosity, but since temperature of the oil bath during the liquid metal granulation goes up, the oil automatically acquires a satisfactory viscosity even if its viscosity at the room temperature is high.
There are optimum oils available on the market which are quite suitable to work as coolant "Quenching oils/Mineral oils" in the said magnesium granulation process, but unfortunately, these oils are rather difficult to remove from the final product down to the required low concentrations. The reason is that for effective deoiling through the said heating process, the oil should have relatively high vapour pressure at moderate temperatures and it should preferably have low viscosity at room temperature. The latter one is of importance in the centrifuge treatment and it affects the content of remaining oil in the centrifuged material. Unfortunately, the fact is that the oil properties which are optimum from the point of view of the deoiling process are less attractive in oils to be used as a coolant in the said granulation process. The said quenching- and mineral-oils have low vapour pressure and high boiling point with the result that the granules require heating to rather high temperatures. Heating of magnesium- and magnesium alloy granules to high temperatures is dangerous from the point of view of safety and product quality. In addition, use of rather high temperatures make the whole deoiling process complex, energy consuming and costlier. The above problems give rise to two options; 1) to make a compromise and choose an oil with properties lying in between the quenching oil and light oil. 2) Another way is to choose a deoiling process which is insensitive to or which can accept the properties of the optimum cooling oil, for example a solvent extraction process involving risk of safety and serious environmental problems. In any case, one would be forced to deviate from the optimum and most practical and economical solutions.